Mitogen-activated protein kinase


A mitogen-activated protein kinase is a type of serine/threonine-specific protein kinases involved in directing cellular responses to a diverse array of stimuli, such as mitogens, osmotic stress, heat shock and proinflammatory cytokines. They regulate cell functions including proliferation, gene expression, differentiation, mitosis, cell survival, and apoptosis.
MAP kinases are found in eukaryotes only, but they are fairly diverse and encountered in all animals, fungi and plants, and even in an array of unicellular eukaryotes.
MAPKs belong to the CMGC kinase group. The closest relatives of MAPKs are the cyclin-dependent kinases.

Discovery

The first mitogen-activated protein kinase to be discovered was ERK1 in mammals. Since ERK1 and its close relative ERK2 are both involved in growth factor signaling, the family was termed "mitogen-activated". With the discovery of other members, even from distant organisms, it has become increasingly clear that the name is a misnomer, since most MAPKs are actually involved in the response to potentially harmful, abiotic stress stimuli. Because plants cannot "flee" from stress, terrestrial plants have the highest number of MAPK genes per organism ever found. Thus the role of mammalian ERK1/2 kinases as regulators of cell proliferation is not a generic, but a highly specialized function.

Types

Most MAPKs have a number of shared characteristics, such as the activation dependent on two phosphorylation events, a three-tiered pathway architecture and similar substrate recognition sites. These are the "classical" MAP kinases. But there are also some ancient outliers from the group as sketched above, that do not have dual phosphorylation sites, only form two-tiered pathways, and lack the features required by other MAPKs for substrate binding. These are usually referred to as "atypical" MAPKs. It is yet unclear if the atypical MAPKs form a single group as opposed to the classical ones.
The mammalian MAPK family of kinases includes three subfamilies:
  1. Extracellular signal-regulated kinases
  2. c-Jun N-terminal kinases
  3. p38 mitogen-activated protein kinases
Generally, ERKs are activated by growth factors and mitogens, whereas cellular stresses and inflammatory cytokines activate JNKs and p38s.

Activation

Mitogen-activated protein kinases are catalytically inactive in their base form. In order to become active, they require phosphorylation events in their activation loops. This is conducted by specialized enzymes of the STE protein kinase group. In this way protein dynamics can induce a conformational change in the structure of the protein via long-range allostery.
In the case of classical MAP kinases, the activation loop contains a characteristic TxY motif that needs to be phosphorylated on both the threonine and the tyrosine residues in order to lock the kinase domain in a catalytically competent conformation. In vivo and in vitro, phosphorylation of tyrosine oftentimes precedes phosphorylation of threonine, although phosphorylation of either residue can occur in the absence of the other.
This tandem activation loop phosphorylation is performed by members of the Ste7 protein kinase family, also known as MAP2 kinases. MAP2 kinases in turn, are also activated by phosphorylation, by a number of different upstream serine-threonine kinases. Because MAP2 kinases display very little activity on substrates other than their cognate MAPK, classical MAPK pathways form multi-tiered, but relatively linear pathways. These pathways can effectively convey stimuli from the cell membrane to the nucleus or to many other subcellular targets.
In comparison to the three-tiered classical MAPK pathways, some atypical MAP kinases appear to have a more ancient, two-tiered system. ERK3 and ERK4 were recently shown to be directly phosphorylated and thus activated by PAK kinases. In contrast to the classical MAP kinases, these atypical MAPKs require only a single residue in their activation loops to be phosphorylated. The details of NLK and ERK7 activation remain unknown.
Inactivation of MAPKs is performed by a number of phosphatases. A very conserved family of dedicated phosphatases is the so-called MAP kinase phosphatases, a subgroup of dual-specificity phosphatases. As their name implies, these enzymes are capable of hydrolyzing the phosphate from both phosphotyrosine and the phosphothreonine residues. Since removal of either phosphate groups will greatly reduce MAPK activity, essentially abolishing signaling, some tyrosine phosphatases are also involved in inactivating MAP kinases.

Signaling cascades

As mentioned above, MAPKs typically form multi-tiered pathways, receiving input several levels above the actual MAP kinase. In contrast to the relatively simple, phosphorylation-dependent activation mechanism of MAPKs and MAP2Ks, MAP3Ks have stunningly complex regulation. Many of the better-known MAP3Ks, such as c-Raf, MEKK4 or MLK3 require multiple steps for their activation. These are typically allosterically-controlled enzymes, tightly locked into an inactive state by multiple mechanisms. The first step en route to their activation consists of relieving their autoinhibition by a smaller ligand. This commonly happens at the cell membrane, where most of their activators are bound. That step is followed by side-to-side homo- and heterodimerisation of their now accessible kinase domains. Recently determined complex structures reveal that the dimers are formed in an orientation that leaves both their substrate-binding regions free. Importantly, this dimerisation event also forces the MAP3 kinase domains to adopt a partially active conformation. Full activity is only achieved once these dimers transphosphorylate each other on their activation loops. The latter step can also be achieved or aided by auxiliary protein kinases. Once a MAP3 kinase is fully active, it may phosphorylate its substrate MAP2 kinases, which in turn will phosphorylate their MAP kinase substrates.

In animals

The ERK1/2 pathway of mammals is probably the best-characterized MAPK system. The most important upstream activators of this pathway are the Raf proteins, the key mediators of response to growth factors ; but other MAP3Ks such as c-Mos and Tpl2/Cot can also play the same role. All these enzymes phosphorylate and thus activate the MKK1 and/or MKK2 kinases, that are highly specific activators for ERK1 and ERK2. The latter phosphorylate a number of substrates important for cell proliferation, cell cycle progression, cell division and differentiation
In contrast to the relatively well-insulated ERK1/2 pathway, mammalian p38 and JNK kinases have most of their activators shared at the MAP3K level. In addition, some MAP2K enzymes may activate both p38 and JNK, while others are more specific for either JNK or p38. Due to these interlocks, there are very few if any stimuli that can elicit JNK activation without simultaneously activating p38 or reversed. Both JNK and p38 signaling pathways are responsive to stress stimuli, such as cytokines, ultraviolet irradiation, heat shock, and osmotic shock, and are involved in adaptation to stress, apoptosis or cell differentiation. JNKs have a number of dedicated substrates that only they can phosphorylate, while p38s also have some unique targets, ensuring the need for both in order to respond to stressful stimuli.
ERK5 is part of a fairly well-separated pathway in mammals. Its sole specific upstream activator MKK5 is turned on in response to the MAP3 kinases MEKK2 and MEKK3. The specificity of these interactions are provided by the unique architecture of MKK5 and MEKK2/3, both containing N-terminal PB1 domains, enabling direct heterodimerisation with each other. The PB1 domain of MKK5 also contributes to the ERK5-MKK5 interaction: it provides a special interface through which MKK5 can specifically recognize its substrate ERK5. Although the molecular-level details are poorly known, MEKK2 and MEKK3 respond to certain developmental cues to direct endothel formation and cardiac morphogenesis. While also implicated in brain development, the embryonic lethality of ERK5 inactivation due to cardiac abnormalities underlines its central role in mammalian vasculogenesis. It is notable, that conditional knockout of ERK5 in adult animals is also lethal, due to the widespread disruption of endothelial barriers. Mutations in the upstream components of the ERK5 pathway are thought to underlie cerebral cavernous malformations in humans.

In fungi

MAPK pathways of fungi are also well studied. In yeast, the Fus3 MAPK is responsible for cell cycle arrest and mating in response to pheromone stimulation. The pheromone alpha-factor is sensed by a seven transmembrane receptor. The recruitment and activation of Fus3 pathway components are strictly dependent on heterotrimeric G-protein activation. The mating MAPK pathway consist of three tiers, but the MAP2 and MAP3 kinases are shared with another pathway, the Kss1 or filamentous growth pathway. While Fus3 and Kss1 are closely related ERK-type kinases, yeast cells can still activate them separately, with the help of a scaffold protein Ste5 that is selectively recruited by the G-proteins of the mating pathway. The trick is that Ste5 can associate with and "unlock" Fus3 for Ste7 as a substrate in a tertiary complex, while it does not do the same for Kss1, leaving the filamentous growth pathway to be activated only in the absence of Ste5 recruitment.
Fungi also have a pathway reminiscent of mammalian JNK/p38 signaling. This is the Hog1 pathway: activated by high osmolarity or a number of other abiotic stresses. The MAP2 kinase of this pathway is called Pbs2, the dedicated MAP3 kinases involved in activation are Ssk2 and SSk22. The system in S. cerevisiae is activated by a sophisticated osmosensing module consisting of the Sho1 and Sln1 proteins, but it is yet unclear how other stimuli can elicit activation of Hog1. Yeast also displays a number of other MAPK pathways without close homologs in animals, such as the cell wall integrity pathway or the sporulation pathway.